The present invention relates to apparatuses and methods for measuring and controlling the amount of multiple coating materials applied to a substrate, and in particular to an apparatus and method for monitoring and regulating the basis weight of multiple coating materials containing different compositions applied to a paper sheet, such as paperboard.
It is often desirable to coat paperboard with different types of coating materials to make a white printable surface. Although the paperboard can be coated with a wide variety of materials, it is often first coated with a base coat, which may be composed of a paint-like substance of principally calcium carbonate (CaCO.sub.3), a latex binder and possibly some clay. A top coat is then applied which may be composed of principally clay, a latex binder and possibly some CaCO.sub.3. Both the CaCO.sub.3 and clay serve as white pigments. CaCO.sub.3 is the less expensive pigment, but does not enhance the printability of the paperboard as well as clay. Therefore, CaCO.sub.3 is preferred as the principal component of the base coat and clay is preferred as the principal component of the top coat on which the printing takes place. The coatings provide a white surface for packaging boxes, such as cereal boxes, shoe boxes, and the like. Alternatively, or in addition, such coatings also may be intended to render paperboard waterproof.
These coatings may be applied to the paperboard as part of the papermaking process in a paper mill. Alternatively, previously manufactured paperboard may be supplied from large rolls of paperboard to separate coating machines, called "coaters." In either process, the uncoated paperboard is usually fed sequentially to the coaters in sheets that are 15 feet or more in their "cross-directional" width (i.e., the direction transverse to the direction of movement of the paper through the paperboard machinery).
It is often desirable to apply a uniform "basis weight" (i.e., mass of the coating material on a unit of surface area of the sheet) of coating to the substrate. For example, the printability of paperboard is improved by a uniform layer of gloss coating which may include relatively expensive materials, such as latex and/or TiO.sub.2. Of course, a uniform coating layer avoids wasting such materials. In some processes, the uniformity of the coatings should be controlled to a fraction of a gram/m.sup.2.
Various complex coaters have been designed to handle the relatively large cross-directional width of paperboard and the requirement for a uniform coating. One type of coater, a blade coater, includes a rotating backing drum located adjacent to one side of a moving sheet and a flexible blade located adjacent to the opposite side of the sheet. The drum and blade edge form a narrow slot through which the sheet passes and retain a pool of coating material which coats the sheet as it passes through the slot. The blade edge presses the sheet passing through the slot to remove excess coating.
The distance from the drum to the blade edge is a critical factor; thus,the drum must be fabricated and installed to high tolerances. To control the thickness of the coating applied to a sheet, coaters provide actuators for adjusting the pressure of the blade edge against the coated sheet and/or the position of the blade edge relative to the drum. The blade is usually made of a thin steel member which may be slightly bent or flexed. Actuators are installed at intervals along the length of the blade so that each actuator controls the pressure applied by the blade in the vicinity of the actuator, and therefore, controls the amount of coating applied to the sheet. The cross-directional length of the blade in the vicinity of each actuator is referred to as a "slice." U.S. Pat. No. Re. 33,741, formerly U.S. Pat. No. 4,732,776 to Boissevain, assigned to Measurex Corporation, describes a coater including this type of coating blade actuator. This reissue patent is incorporated herein by reference.
Local variations in sheet temperature and thickness, and possibly other factors, if not compensated for, will tend to produce uneven coatings. Therefore, it is important to have the ability to measure the amount of coating at various locations on the sheet and to control the pressure of the blade against the sheet at a plurality of cross-directional positions based on the measurements.
U.S. Pat. No. 4,957,770 to Howarth, assigned to Measurex Corporation, describes a coating sensor for measuring the basis weight of a coating on paper by detecting the amount of a single coating component. This patent is incorporated herein by reference. However, applicants are unaware of any apparatus and method, other than their own, which uses a single sensor to measure the amount of multiple coating components to determine the amount of multiple coatings on a substrate. One of the most difficult aspects of multiple coating control is obtaining an accurate measurement of the amount of each of the coating layers applied to a sheet, particularly if the coating must be measured to an accuracy in fractions of a gram/m.sup.2.
However, there is a scheme for measuring the amount of two coatings applied to a paper sheet by use of six sensors. Under this scheme, the paper sheet first passes through a first coater which applies a base coat and then travels through a stack of heating drums which dry the base coat. The sheet then travels through a second coater which applies a top coat on the base coat and then travels through a stack of heating drums which dry the top coat.
A first basis weight sensor and a moisture sensor are located upstream of the first coater. The first basis weight sensor measures the basis weight of the uncoated sheet (i.e., paper fiber and moisture). A beta ray basis weight sensor will determine the basis weight of a sheet by detecting the amount of beta rays transmitted through the sheet. An infrared moisture sensor will determine the moisture content of the sheet by detecting the amount of infrared radiation either transmitted through or reflected from the sheet in a spectral band centered about a moisture absorption peak.
A second basis weight sensor and a moisture sensor are located downstream of the first coater and upstream of the second coater. The second basis weight sensor measures the total basis weight of the sheet exiting from the first coater, including the basis weight of the paper fibers, the base coat and the moisture absorbed by the paper fibers.
The amount of paper fiber forming the sheet can be determined by subtracting the basis weight of the moisture from the basis weight of the uncoated sheet. Similarly, by subtracting the basis weight of the moisture of the base-coated sheet from the basis weight of the base coated sheet, the basis weight of the base coat and paper fiber (without the moisture) can be determined. Finally, by subtracting the basis weight of paper fiber from the basis weight of the base coated sheet (without moisture), the basis weight of the base coat can be determined.
A process control computer can be used to compare the measured basis weight of the base coat at each slice with a desired value and to generate a corresponding signal to control the coating blade actuators at each slice to achieve the desired base coat profile. However, as explained below, the scheme still requires a third basis weight sensor and a moisture sensor to determine the basis weight of the top coat.
The third basis weight sensor and a moisture sensor are located downstream from both the first and the second coater. The third basis weight sensor measures the total basis weight of the sheet exiting from the second coater, including the basis weight of the paper fibers, the base coat, the top coat and the moisture content. The third moisture sensor is necessary when the heating drums are used to dry the base coat (and remove moisture from the paper) because the moisture content of the paper exiting the first coater no longer equals the moisture content of the paper exiting the second coater.
The basis weight of the top coat is calculated in the following manner. First, the measured value of the moisture content of the paper exiting the second coater is subtracted from the total basis weight of the sheet measured by the third basis weight sensor. This calculation provides the total basis weight of the top and base coat and paper fiber (without the moisture). The basis weight of the top coat is then determined by subtracting the basis weight of the paper fiber and base coat from the total basis weight of the sheet exiting the second coater.
The process control computer can then be used to compare the basis weight of the top coat at each slice across the width of the sheet with desired value(s) and to generate a corresponding signal to control coating blade actuators at each slice to achieve the desired top coat profile.
However, the above-described scheme requires six sensors which must be calibrated and kept in good working order. Moreover, the error inherent in the measurements obtained from each of these six sensors will propagate through the mathematical calculations necessary to determine the basis weight of the top and base coats.
Another scheme for measuring the amount of coating material applied to a sheet requires the irradiation of the coated sheet with very high energy x-rays which excite the atoms in the coating so that such atoms fluoresce. The fluorescing atoms emit x-rays having wavelengths unique to the elements in the coating. An x-ray can determine the amount of coating by the intensity of the fluorescence at the characteristic wavelengths.
In yet another technique, portions of the sheet are irradiated with x-rays and the intensity of the x-rays transmitted through the sheet is detected. However, x-rays are absorbed by the wood pulp fibers, moisture or mineral filler material frequently used in paper sheet. Because the transmission of x-rays is not affected by the coating material alone, sensors must be positioned before and after the coater; and the difference in transmission of the x-rays through the coated and uncoated portions of the sheet measured and related to the amount of coating material applied to the sheet. This technique requires multiple x-ray sources and sensors. In addition, the error inherent in measurements made by each sensor may contribute to the error in the determined amount of coating.
Accordingly, the present invention provides a simpler, less expensive and more accurate sensor, system and method for measuring and controlling the amount of multiple coating layers applied to a substrate.